This invention relates to data transmission over multimode optical fiber and, more particularly, to a system and method for reducing inter-symbol interference by compensating for differential mode dispersion.
The majority of optical fiber that has been installed for high-speed data transmission is predominately multimode optical fiber. The main source of bit errors in a multimode fiber (MMF) transmission system is inter-symbol interference (ISI) caused by differential mode dispersion (DMD). DMD imposes an upper limit on the bandwidth-distance product of the MMF.
Specifically, DMD limits the bandwidth-distance product of an MMF to approximately 300 MHz-km. A bandwidth-distance product of 300 MHz-km will limit the transmission distance of an MMF to approximately 30 meters at a data rate of 10 Gb/s.
Prior art methods to overcome the low bandwidth-distance product exhibited by MMFs include: (1) selective excitation of a limited number of modes, as described in L. Raddaz et al. xe2x80x9cAn Experimental and Theoretical Study of the Offset Launch Technique for the Enhancement of the Bandwidth of Multimode Fiber Links,xe2x80x9d IEEE J. Lightwave Technology, pages 324-331 (1998); (2) alternative modulation schemes, such as those described by T. K. Woodward et al, xe2x80x9c1-Gb/s BTSK Transmission at 850 nm Over 1 km of 62.5-xcexcm Core Multimode Fiber Using a Single 2.5 GHz Subcarrier,xe2x80x9d IEEE Photon: Technology Letters, Vol. 11, pp. 382-384 (1999), E. J. Tyler et al., xe2x80x9cTransmission of a Single 2.5 Gb/s Subcarrier Modulated Channel Over 300 m 6.25 xcexcm Multimode Fiber,xe2x80x9d IEEE LEOS annual meeting paper, TuBB4 (2000), and R. Taborek, xe2x80x9cMultilevel Serial PMD Update,xe2x80x9d presented at IEEE 802.3 HSSG, Kauai, Hi. (November 1999); and (3) wavelength division multiplexing techniques, such as those described in B. Lemoff et al., xe2x80x9cLow-Cost MM WDM for the Local Area Network,xe2x80x9d IEEE Summer Topical Meeting, WDM Components Technologies, Montreal, Quebec, Canada paper WC3 (1997) and Q. Xiang, Y. Zhao, Y. Chai, F. S. Choa, xe2x80x9cSchematic Studies of 10 Gb/s Transmission Over Multimode Fibers,xe2x80x9d Proc. IEEE LEOS Annual Meeting, paper TUR 2 pp. 271-272 (November 1999).
Selective excitation of a limited number of modes by offset launching can depress lower order modes and obtain a lower ISI. However, the distance/bit-rate product is limited to approximately 500 Gd.m/s. Alternative modulation schemes and wavelength multiplexing techniques can potentially work, however, they are very costly approaches.
In an exemplary embodiment of the invention, a system for compensating for unwanted modes in multimode fiber transmissions comprises a detector that receives an optical signal transmitted through a multimode optical fiber and converts the transmitted optical signal to a detector signal, and an adaptive equalizer that generates an adaptive equalizer signal that, when combined with the detector signal, compensates for unwanted modes in the detector signal.
The present invention also provides a system for compensating for differential mode dispersion effects in multimode fiber transmissions, comprising a detector that receives an optical signal transmitted through a multimode optical fiber and converts the transmitted optical signal to a detector signal, a decision element that receives the detector signal, determines a symbol represented by the detector signal, and outputs the determined symbol as an output signal, an adaptive equalizer that receives at least a portion of the output signal as a feedback signal and generates an adaptive equalizer signal based on the feedback signal, and an adder that combines the detector signal with the adaptive equalizer signal, thereby compensating for unwanted modes in the detector signal caused by differential mode dispersion in the multimode optical fiber.
The present invention also provides a method of compensating for unwanted modes in multimode fiber transmissions, comprising the steps of converting an optical signal transmitted through a multimode optical fiber to an electrical signal, generating an equalizer signal that, when combined with the electrical signal, reduces unwanted modes in the electrical signal, and combining the equalizer signal with the electrical signal.